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G8MNY > TECH 13.07.03 00:47l 57 Lines 2608 Bytes #999 (0) @ WW
BID : 5400_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: Recovering OLD NICADS
Path: DB0FHN<DB0RGB<OK0PPL<RZ6HXA<IK1ZNW<GB7CRV<GB7CIP
Sent: 030712/2226Z @:GB7CIP.#32.GBR.EU #:5400 [Caterham] $:5400_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
Old Nicads batterys usually end up with several cells being short circuit, as
they suffer from crystal growth in the electrolyte. To reclaim the cell before
charging, (as charging will not remove the short) a very high current pulse
must be used.
With a sealed battery pack this can blow any internal safety fuse! So it is
best done when U have opened the pack. The good cells will withstand the high
current OK, but having them in series can reduce the current depending on how
you are applying it, if you have a really bad cell.
Current up to 10xC (eg 5A on a 500mAH AA cell, 40A on a 4AH D cell) can be
safely applied until the pack warms, (how fast chargers detect 70% charge &
drop back to trickle).
For more difficult cells, current over 10xC may be required, this can destroy
the internal cell wires, but you have nothing to loose! Charge up a 10,000uF
to 40V & connect to cell (computer must not be nearby!) with thick wires, the
100A pulse usually clears the short after a few pulses.
Once all the cells are over 1V, then trickle charge (Cx0.1) until warm or 14
hours. A few cycles of discharge & charge will bring back old cells to 100%.
If the capacity is less than 100% after a few cycles or there is excessive volt
age (>2V) across a cell when charging, it is O/C or dry & will need to be
replaced.
From: G8TBF@GB7NND.#23.GBR.EU
To : G8MNY@GB7CIP.#32.GBR.EU
Hi John,
Here is a couple more bits of info for your collection:-
Tony, G8TBF says cells going short due to crystallization is not due to the
electrolyte but the cadmium electrode; cadmium has a property of 'cold
crystallization' (like Tin & Zinc) and can form 'needle' type spikes that
bridge the cell.
The way to prevent this is to occasionally give the cells a long charge at a
low current (<= 10 hour rate). This dissolves any 'needles' and re-plates the
surfaces.
The other effect of the cadmium crystallization is the 'memory effect' - this
happens when cells are not fully discharged for long periods, and is caused by
the 'deeper' parts of the cadmium plate crystallizing.
The effect is that once the amorphous metal on the surface of the plate has
been dissolved during discharge & the crystalline part is exposed, the cadmium
is much less reactive - a crystalline structure simply does not break down as
easily as amorphous metal.
The cure for 'memory effect' is to deep-discharge the cell, by connecting
(e.g.) a 10 ohm resistor across it and leave it until absolutely dead.
Normal recharging will then give full capacity & no 'memory'.
/QSL
73 De John, G8MNY @ GB7CIP
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